The invention relates to headgears and mounting apparatuses, specifically devices that enable the rapid and reliable placement of sensors on a subject's head. The focus of this patent is for dry electrode EEG applications, although the invention is broadly applicable to placing any type sensor or transducer on the head of a subject.
Conventional recording of EEG signals predominantly involves the use of wet electrodes that utilize a gel for conduction to the subject's scalp. The wet electrodes can be affixed to the skin either with glue or placed in an elastic cap. In many cases, abrasion of the scalp is necessary. The high conductivity of the gel allows electrical conductivity to permeate through hair and any physical gaps between the surface of the electrode and the surface of the scalp is filled and buffed by the gel's liquidity. As a result, wet electrodes offer a secure, low-impedance electrical connection between the subject and the recording instruments with minimal need for advanced mechanics.
However, the use of electrolytic gels, adhesives and scalp abrasion is often time consuming, irritating and uncomfortable for the subject. In response, dry electrodes, which do not require conductive gels, adhesives, or scalp preparation, have been explored as an alternative. In practice, dry electrodes suffer from numerous usability issues. Although acquiring signals on bare skin (e.g., forehead) is straightforward, most EEG setups also require electrodes to be placed across the entire head and especially over areas with hair. With no gel buffer, dry electrodes depend on mechanically stable and adjustable sensor mounts to secure the sensor on the surface of the subject's head.
Challenges with dry EEG headgears involve the need to conform to the many variations in human head shape and size with a standard design that can be used by all subjects. For a dry EEG headgear to be useful, it must also be simple to apply and remove. The basic approach to constructing a dry EEG headgear is based upon adapting the standard elastic EEG cap, exemplified by Gevins et. al. in U.S. Pat. No. 4,967,038. The elasticity in the cap enables it to stretch and cover different sized heads. However, elastic caps suffer from a numerous problems. Although elastic caps are generally flexible, the generic ‘balloon-like’ shape does not fully conform well to the exact contours of different head shapes (e.g., ‘boxy’ heads or dimples), leading to areas where the cap is overly tight and other areas where the cap is loose. In addition, the closed nature of the cap makes adjusting the electrodes difficult in cases where the electrodes are in the wrong position or poorly contacting the scalp. Finally, because the elastic cap has no rigid supports, individual electrodes inside the cap are prone to tipping and misalignment during application.
As a result, more sophisticated designs have been explored that use mechanical headgears with mechanisms to individually place sensors on the head for better performance and reliability. One example is found in U.S. Pat. No. 8,103,328 by Turner et al. where each sensor is mounted on a spring-loaded assembly at the tip of an arm connected to a hinge. The hinging mechanism in the arm helps orient the sensor to the surface of the head and the arm generates pressure that connects the sensor to the head. This system is effective at providing optimized and individual tension for each of the sensors in the array but the overall design is complex and bulky. Each headgear contains multiple joints, springs and other moving parts making the system heavy and expensive.
For a simpler and less expensive design, Trewartha et al. (WO 2008/109699 A2) shows a headgear that contains only a few moving parts. In this design, most of the hinges and springs are replaced with solid arms. Only two pivots are present for rotating two major groups of sensors. Plastic deformation of the arms generates tension without the need for other mechanics of moving parts. However, the design shown by Trewartha imposes severe geometric constraints, especially for EEG applications which involve placement of the sensors at specific locations around the scalp. The headgear includes a left band and a right band that clamp on to the head near a user's temples. While this is effective in securing the headgear to the user, the arrangement biases the locations of the electrodes to the sides of the head—areas that are particularly prone to muscular artifacts. Placement of sensors on the upper half of the head and the back of the head, which is necessary for many research and medical EEG applications, cannot be readily accommodated with this setup.